1. Field of the Invention
The present invention relates to an optical pickup apparatus for writing or reading an information signal such as a video signal, an audio signal and a computer data signal to/from an optical recording medium such as an optical disc.
2. Description of the Related Art
Studies of a high density recording of an information signal including not only an audio signal but also a video signal or computer data on an optical disc are advancing. For example, in case of recording an information signal including a video signal of a movie of about two hours to an optical disc having a size of a well-known compact disc on which only an audio signal has been recorded, recording density of the optical disc has to be set to an extremely high density. In order to accomplish high density recording, a method of narrowing an interval between tracks on each of which the information signal is recorded is considered. When the interval between tracks, that is, a track pitch is decreased, there is a problem such that a crosstalk amount which is mixed in the information signal increases, thereby deteriorating a quality of a reproduced information signal. Especially, when the optical disc is inclined to an optical axis, that is, when there is a tilt in the optical disc, the crosstalk amount from one of neighboring tracks increases and a situation such that the signal cannot be preferably reproduced occurs.
The crosstalk occurring due to the existence of the tilt of the optical disc is principally caused by a coma aberration shown by the following equation (1). ##EQU1## where, t : disc thickness NA: numerical aperture of an objective lens
.theta.: tilt angle of the disc PA1 n: refractive index of the disc PA1 .lambda.: reproduction wavelength
It will be understood from the equation (1) is that an occurrence amount of the coma aberration is proportional to the cube of the numerical aperture NA of the objective lens and is inversely proportional to the reproduction wavelength .lambda. even in the case of the same disc tilt angle .theta.. In an optical disc such as a DVD (digital video disc) on which a digital information signal has been recorded, since it is necessary to increase the numerical aperture NA to, for example, 0.6 in order to realize the high density recording, a method of setting the reproduction wavelength .lambda. to a short wavelength. However, when the reproduction wavelength .lambda. is set to be shorter than a present value, it is beyond the limit of the disc manufacturing technique. Consequently, there is no way except a decrease in numerical aperture NA of the objective lens and this obstructs the realization of a high recording density of the optical disc. It is, therefore, difficult to reduce the coma aberration due to the set values of the numerical aperture NA and reproduction wavelength .lambda. for the purpose of the realization of the high recording density of the optical disc.
In a conventional pickup apparatus, a tilt sensor for detecting an inclination of the disc is provided in order to reduce the crosstalk, and a tilt servo control for controlling the inclination of the optical axis to the optical disc in accordance with a tilt detection amount detected by the tilt sensor is executed.
FIG. 1 shows a tilt sensor which is used for a conventional pickup apparatus. In the tilt sensor, a light emitting diode 11 generates a beam for detecting a tilt of a disc to a disc 12. Photodetectors 13 and 14 are arranged at both side positions of the light emitting diode 11 in the radial direction of the disc and receive a light reflected from the disc 12, respectively. Photosensitive level signals of the photodetectors 13 and 14 are supplied to a differential amplifier 15 and a difference between the level signals is obtained. For example, when the disc 12 is tilted so that the right side thereof goes down in the diagram, the output level of the photodetector 13 increases and the output level of the photodetector 14 decreases, so that the tilt of the disc can be detected from the level difference. An output signal of the differential amplifier 15 is supplied to a driving mechanism for adjusting a tilt of a whole pickup and the tilt of the whole pickup is automatically adjusted so as to correct the tilt detected by a disc tilt detector.
The tilt sensor, however, requires a space of a certain size, so that it cannot be arranged at a converging position of a reproducing beam. The tilt sensor, consequently, detects the tilt at a position different from a reproducing (recording) position. According to such an indirect method, it cannot be said that accurate correction is executed. A change in output due to aging changes of the tilt sensor, pickup, and the like also disturbs the accurate correction.
This results in a problem especially in playing an optical disc of a high density recording which requires the correction of the crosstalk as a prerequisite.